His goal was to kill the malware. Registering the (unclaimed) domain in the binary was supposed to be step 0 to this. Such a domain would almost certainly act as a control center of some sort. You can claim it while it's still available and analyze the malware or even just the traffic reaching your domain to try and neuter it. Even if there's no killswitch, maybe sending invalid data will cause the malware to malfunction and effectively stop its spread for example.

The fact that just registering the domain killed WannaCry wasn't expected, but his intent was to kill the virus from the start, that's no accident.

isn't the first thing anyone would do when coming across such a domain in a malware binary to check it is claimed (and if not then who here wouldn't register it (even just if to see what happens)?) I mean we can argue over the semantics of accidental, but imo you can't accidentally register a domain?